Silver conductive pastes which are widely used as electrically conductive materials for electronic circuits, have several drawbacks such as their high production costs and their tendencies of electrochemical migration in which electrode materials are ionized and move toward the counter electrode.
To remove these drawbacks, there is proposed the use of copper conductive paste instead of silver conductive paste. Since, the copper paste, however, is easily oxidized, it requires an anti-oxidant treatment such as follows.
Method (i): The addition of the reductive substance capable of reducing oxide cover film of copper powder or the use of the reductive resin [the Japanese Patent Open-laid Applications (hereinafter referred to as JPOA) Nos. 81/103260, 82/96401, 82/34060].
Method (ii) Coating of copper powder with silver (the JPOA Nos. 78/135495, 81/8892).
Since, however, in the Method (i), the copper powder itself is not subjected to anti-oxidant treatment, the copper powder or the paste containing such non-treated copper powder becomes unstable after it is stored for a long time. In the presence of a reductive substance, the cured film has insufficient resistivity against heat and the wet. When the conductor film is formed by the use of the reductive resins of the restricted kinds, it is hardly used in the multi-functional devices.
On the other hand, the Method (ii) requires much amount of silver (20% by weight in the case of the JPOA No. 78/135495, 10% by volume in the case of the JPOA No. 81/8892). Therefore, the composite copper powder becomes expensive and is lacking in anti-migratory action as well.